Field of the Invention
The present invention relates to inspection apparatus and methods usable, for example, to perform metrology in the manufacture of devices by lithographic techniques.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field.
Examples of known scatterometers often rely on provision of dedicated metrology targets. For example, a method may require a target in the form of a simple grating that is large enough that a measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In so-called reconstruction methods, properties of the grating can be calculated by simulating interaction of scattered radiation with a mathematical model of the target structure. Parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.
In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Examples of dark field imaging metrology can be found in numerous published patent applications, such as for example US2011102753A1 and US20120044470A. Multiple gratings can be measured in one image, using a composite grating target. The known scatterometers tend to use light in the visible or near-IR wave range, which requires the grating to be much coarser than the actual product structures whose properties are actually of interest. Such product features may be defined using deep ultraviolet (DUV) or extreme ultraviolet (EUV) radiation having far shorter wavelengths. Unfortunately, such wavelengths are not normally available or usable for metrology. Product structures made for example of amorphous carbon may be opaque to radiation of shorter wavelength.
On the other hand, the dimensions of modern product structures are so small that they cannot be imaged by optical metrology techniques. Small features include for example those formed by multiple patterning processes, and/or pitch-multiplication. Hence, targets used for high-volume metrology often use features that are much larger than the products whose overlay errors or critical dimensions are the property of interest. The measurement results are only indirectly related to the dimensions of the real product structures, and may be inaccurate because the metrology target does not suffer the same distortions under optical projection in the lithographic apparatus, and/or different processing in other steps of the manufacturing process. While scanning electron microscopy (SEM) is able to resolve these modern product structures directly, SEM is much more time consuming than optical measurements. Other techniques, such as measuring electrical properties using contact pads is also known, but it provides only indirect evidence of the true product structure.
The inventor has considered whether the techniques of coherent diffraction imaging (CDI), combined with radiation of wavelength comparable with the product structures of interest, might be applied to measure properties of device structures. CDI is also known as lensless imaging, because there is no need for physical lenses or mirrors to focus an image of an object. The desired image is calculated synthetically from a captured light field.
Using lensless imaging to retrieving images of two-dimensional structures has been demonstrated. For example, two-dimensional structures have been obtained for both transmission and reflection geometries, achieving 22 nm transverse spatial resolution (“Lensless diffractive imagine with ultra-broadband table-top sources: from infrared to extreme-ultraviolet wavelengths” M. D. Seaberg et al., Optics Express 19, 22470 (2011)). Additionally, it has been demonstrated that it is possible to image through thin metal layers using lensless imaging at extreme ultraviolet (EUV) wavelengths (S. Witte et al., Light: Science & Applications, e163 (2014)).
However, using these methods, it is not possible to obtain information regarding the three-dimensional properties, in particular depth information, of a given structure. In effect, it is not possible to obtain information regarding patterned layers inside a structure. Given that product structures typically comprise several patterned layers, this is a drawback. Furthermore, existing inspection methods, such as SEM, also do not provide any depth information regarding multi-layer structures.